Method for biological purification of effluents using biofilm supporting particles

ABSTRACT

The invention concerns a method for biological purification of effluents in mixed cultures using micro-organisms whereof part at least is fixed on solid supports. The invention is characterized in that said supports are activated so as to generate a turbulence in the reaction medium, the intensity of which is such that it reduces the production of biological sludge, the materials constituting said micro-organism supports being abraded and cleaned, while being retained in said reaction medium.

FIELD OF THE INVENTION

The present invention relates to a method for the biologicalpurification of wastewater employing a hybrid culture system usingbiofilm support particles. It also relates to a reactor or equipment forimplementing such a method.

BACKGROUND OF THE INVENTION

It is known that the purification of municipal and industrial wastewateris often carried out biologically. In recent decades, processes havepassed from those using free microorganism cultures to processes withcultures fixed on specific growth media for the purpose of reducing thesize of purification plants.

Fixed cultures are employed either as a fixed bed, that is to say amicroorganism growth medium is stationary in the reactor, or as a movingbed, in which case the support materials are small elements that canmove freely in the zone of contact with the polluted water. Thesesupport elements may be moved either by mechanical stirring or byinjecting a liquid, or else by injecting a gas, especially air (this airpossibly being, for example, the air needed for the microorganisms tooperate when they are aerobic).

The creation and maintenance of a certain level of turbulence in thereaction medium are useable for continuously abrading and cleaning thesupport material for the microorganisms, this turbulence furthermoremaking it possible to limit the accumulation of fixed biological sludge.Such turbulence may be created, for example, by the intensity of the gasinjected into the medium. Reference may be made in this regard to EP-A-0549 443.

If it is desired to treat the pollution due to carbon and to nitrogensimultaneously, it is possible to find advantageous solutions given thatthe materials serve as growth medium for a certain nitrifying biomass,the growth of which is much higher than that in the absence of thesematerials (see EP-A-0 549 443): these are referred to as hybridcultures.

However, these known systems have a number of drawbacks. Thus, in themethod described above, the production of biological sludge is tied tothe normal growth metabolism of the bacteria decontaminating the water.Furthermore, the growth medium materials used are held in place in thereaction chamber either by a retention screen (that lets water through,but not the support material) or by means of a specific separationsystem. The major drawback of screens is their clogging.

BRIEF SUMMARY OF THE INVENTION

Starting with these known systems, the objective of the presentinvention was to solve the following two technical problems:

-   -   prevent clogging of the retention screens positioned at the        outlet for the treated water;    -   reduce the amount of sludge produced, compared with the amount        of sludge produced by conventional methods carrying out the same        biological purification.

These technical problems are solved by a method for the biologicalpurification of wastewater in hybrid cultures employing microorganisms,at least some of which are fixed to solid support elements,characterized in that said support elements are set in motion so as togenerate turbulence in the reaction medium, the intensity of whichturbulence is such that it reduces the production of biological sludge,the materials constituting said microorganism support elements beingsubjected to an abrasion action and to a cleaning action while stillbeing retained in said reaction medium, said materials having a surfacetexture that includes regions protected from the abrasion, allowing thegrowth of a biomass providing the biological activity, and abrasiveregions.

The desirable level of turbulence so as to obtain the best results inimplementing the method according to the invention, as defined above,may be expressed by the energy that is supplied by the aeration and/orstirring means. Preferably, this energy is between 1 and 200 watts percubic meter of reactor and preferably between 2 and 50 watts per cubicmeter of reactor. Such energy levels per cubic meter may be economicallyviable on account of the compact nature of the reactors employed in themethod according to the invention that are defined below.

According to a preferred way of implementing the method defined above,the microorganism support material has one dimension, along any axis,that is between 2 and 50 mm.

As mentioned above, the microorganism support material has a surfacetexture such that the surface has regions protected from abrasion,allowing the growth of a biomass for providing the biological activity,and abrasive regions making it possible, in the presence of a sufficientlevel of turbulence (as defined above), to exert friction on theexternal surfaces of the other particles that are present in thereaction medium.

The subject of the present invention is also a biological reactor forimplementing the method defined above, this reactor being characterizedin that it includes microorganism support retention means, these meansbeing positioned upstream of the means for removing the liquid effluentleaving, after treatment, said reactor, these retention meanscomprising:

-   -   a screen inclined to the vertical at an angle of between 0 and        30° approximately and the separation of the bars of which is        determined so that it lets the water through but not the        microorganism support particles;    -   an air injection rail positioned at the base of said screen and        operating continuously or intermittently so as to flush the        screen; and    -   a deflector panel parallel to said screen and located upstream        of the latter.

In the foregoing, the term “upstream” is understood to mean with respectto the direction of effluent flow from its entry into the reactor to itsdischarge therefrom.

Thus by virtue of the invention, the feature consisting in setting themicroorganism support particles in motion, for example by injecting agas or by mechanical stirring or else by a combination of these twomeans, combined with the feature whereby the constituent material of themicroorganism support particles is retained in the reaction medium,while subjecting said material to an abrasion action and to a cleaningaction, makes it possible, on the one hand, to reduce clogging of thescreens retaining the support material and, on the other hand, to reducethe amount of biological purification sludge normally generated comparedwith a method producing the same purification, this reduction beingaround 2 to 50%.

This is because, since the biological reactor in which the methodaccording to the invention is employed includes an inclined screenprovided with a deflector and with an air injection rail that purges thesurface of the screen, less rapid clogging of the screen is ensured thanthat observed in the reactor vessels according to the prior art. It hasbeen observed that the flow of support materials close to the screen,with an increased velocity because of the presence of the deflector,helps to detach the solid materials liable to be deposited on saidscreen, thus making it possible to reduce the rate of clogging.

It has also been observed, surprisingly, that a certain turbulenceintensity in the reaction medium allows the production of biologicalsludge to be reduced. This phenomenon may be explained by the fact thatthe turbulence within the medium generates friction such that themicroorganisms fixed in the form of a biofilm adopt a particularmetabolism. This is because a very high abrasion intensity means thatcertain microorganisms must synthesize substances for increasing themechanical integrity of the biofilm. When the intensity of the abrasionis high enough so that most of the microorganisms adopt this particularform of metabolism, the growth yield (which is generally defined asbeing the amount of cells produced relative to the amount of pollutingmaterial degraded) decreases considerably. This results in a markeddecrease in the amount of sludge produced compared with operation in theabsence of turbulence.

According to the present invention, the microorganism support materialmust have a large surface compared with the volume that it occupies and,preferably, part of this surface must be protected from the turbulenceand from collisions, as was explained above. Thus, according to theinvention, the surface area of the support material is greater than 100m² per cubic meter of material and abrasive excrescences are provided onthe external surface of said material. Thanks to the latter feature,internal regions are defined that will be able to be colonized bymicroorganisms in an amount sufficient to achieve the desired biologicalpurification. The abrasive external surface may be colonized bymicroorganisms in the form of a biofilm, but the intensity of thestirring and of the turbulence will be such that this biofilm will be inperpetual reconstitution, thereby directing the metabolism of some ofthe microorganisms that carry out the purification toward a particularform of metabolism and thus limiting the production of biologicalsludge.

According to the invention, the microorganism support elementspreferably have one dimension between 2 mm and 50 mm and the constituentmaterial of said support elements is a plastic obtained, for example,from recycled material, for example polyethylene. Examples ofmicroorganism support particles that can be employed in the methodaccording to the present invention will be described below in greaterdetail.

The method according to the present invention may be employed inaerobic, anaerobic or anoxic biological treatment modes or in treatmentsystems operating in a combination of these three modes.

In its application to aerobic purification, the method according to theinvention is characterized in that the microorganism support particlesare set in motion by injecting air or an inert gas to which oxygen hasbeen added, the amount of said gas being determined so as, on the onehand, to ensure biological purification and, on the other hand, toobtain the necessary turbulence intensity.

In the case of an application to anaerobic purification or anoxicpurification, the microorganism support elements are set in motion bythe fermentation gas or by a mechanical stirring system.

In its application to a combined carbon/nitrogen treatment involving twosteps, an anoxic step and an aerobic step, with recycling of the mixedsludge from the aerobic step to the anoxic step, the method according tothe invention may be carried out in one or both of said steps,preferably in the aerobic step so as to immobilize the microorganismsthat oxidize the ammoniacal nitrogen. It is also possible to carry out,in the same tank, the anoxic and aerobic steps, the tank then beingaerated intermittently and the stirring during the anoxic phase beingcarried out by another, especially mechanical, means.

Further features and advantages of the present invention will becomeapparent from the description given below, with reference to theappended drawings that illustrate an example of its implementation thatis devoid of any limiting character.

So as to bring out the advantage afforded by the invention as regardsreducing the production of sludge, an experimental apparatus describedbelow was used, the results from which will be commented upon later. Themeans for retaining the microorganism support materials employed in thereactor according to the invention will be described later.

BRIEF DESCRIPTION OF THE DRAWINGS

In the figures:

FIG. 1 is a diagram showing the experimental apparatus used fordemonstrating the reduction in sludge production thanks to theinvention;

FIGS. 2 a to 2 c are curves that demonstrate the results provided by theinvention as regards elimination of the COD;

FIGS. 3 a and 3 b are curves showing the cumulative amount of sludgeproduced as a function of the cumulative amount of COD eliminated ineach of the two experimental reactor lines used (FIG. 1) and for twodifferent sludge ages;

FIG. 4 is a schematic view showing the retention means employed in thereactor according to the invention;

FIG. 5 is a view, on a larger scale, of a detail of FIG. 4; and

FIGS. 6, 7 a, 7 b and 8 show, schematically, examples of microorganismsupport materials that can be used in the method according to theinvention.

As mentioned above, in order to demonstrate the reduction in biologicalsludge production provided by the method according to the invention, twostrictly identical activated-sludge reactor lines were produced, eachreactor being fed with the same wastewater and operating under the sameoperating conditions. One line constituted the control (it is denotedhereafter by “Control line”) containing no floating biomass supportmaterial, the other line (called hereafter the “test line”) containing afloating growth support material for the biomass, according to theinvention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 therefore shows each of the experimental lines. Each linecomprises a biological reactor 8, a settling tank 10, a pH/temperatureprobe 3 and an oxygen probe 2. The reactor 8 is fed via a pump 5 from astorage tank 4 for municipal wastewater that has undergone primarysettling. Discharge from the reactor takes place via an overflow from aliquid/solid separator 9, to the settling tank 10. The decanted waterleaves the plant while some of the sludge is recycled back into thebiological reactor 8 by means of a recirculation pump 6. The excesssludge is removed by means of a purge 11. Each line includes a computer1 for analyzing the results obtained. The biological reactor 8 isstirred by a mechanical stirrer 7 and by aeration, when the latter is inoperation.

As regards the biomass support material, the reader may refer to the endof the present description, which gives a few nonlimiting examplesthereof.

The Test line operates according to the principle described above.

Table I below indicates the principal characteristics of these tworeactor lines.

TABLE I Principal parameters of the lines Values Volume of the reactor(8) 22 liters Volume of the settling  2 liters tank (10) Plastic supportparticles (Test line), made of polyethylene: density 935 kg/m² meandiameter  3 mm geometry irregular particles Volume fill factor 20% (Testline) Mechanical stirrer (7): 2 marine propellers diameter 10 cm

Table II below gives the operating conditions for the Control and Testlines.

TABLE II Nature of the wastewater Domestic wastewater, after to betreated primary settling, stored at 4° C. and replenished every threedays. Easily assimilatable carbon source supplement (acetate, ethanol,propionate, starch) supplied during the anoxic phase Municipalwastewater (MWW): COD 350–500 mg/l COD/BOD5 1.5 SS 100–150 mg/l NTK 60–90 mg/l N—NH₄  50–75 mg/l Supplement: COD Approximately equivalentto the COD of the MWW (therefore synthetic COD = 50% of the total feedCOD). It is supplied in the anoxic phase to both lines. Volume loadapplied 1 kg COD/m³ · d Mass load applied* Varied between 0.5 and 1 kgcos/kg VSS.d Sludge age Varied between 3 and 8 days Controlledtemperature 16° C. ± 1° C. Aerated phase/nonaerated phase alternation:duration of the 45 min/45 min phases aeration control Dissolved oxygen >3 mg/l

Control line: the biomass in equilibrium is smaller for the Test line.

The two lines operated with a continuous feed of wastewater and with aflow rate making it possible to obtain a mean applied load of 1 kg ofCOD per cubic meter of reactor per day.

The biological reactor 8 operated both with aeration and stirring andwith only stirring. This mode of operation made it possible to alternatethe aerobic phases, ensuring nitrification of the species containingammonia (denoted by N—NH₄ in Table II) present in the wastewater (i.e.their conversion into oxidized species such as nitrites or nitrates),and the anoxic phases for denitrification (i.e. the conversion of theoxidized species into molecular nitrogen).

This mode of operation allowed all of the steps of eliminating thenitrogen contamination to be carried out in the same reactor.

During the aerobic phases, the dissolved oxygen concentration wasmaintained at above 3 mg/l. During the anoxic phases, a certain amountof organic carbon, taken from an external carbon source 12, was added tothe reactor 8 so as to reduce the time needed for the denitrificationstep.

During the experiment, the sludge age (that is to say the ratio of thetotal amount of biological sludge contained in the experimental device,the settling tank included, to the amount of biological sludgeextracted) varied between 3 and 8 days. This parameter was adjusted bythe rate of purge 11 of the biological sludge.

The measurements taken relate to all of the parameters that make itpossible to characterize the effects of the contamination entering andleaving the apparatus: total and soluble chemical oxygen demand,ammoniacal nitrogen N—NH₄, nitrites and nitrates. The amount of sludgeis quantified on the basis of the suspended solids (SS) and of volatilesuspended solids (VSS).

The sludge production is calculated as being the sum of sludge extractedby the purge, the amount of sludge leaving in the decanted effluent andthe accumulation of sludge in the biological reactor (in free form or infixed form).

An apparent biomass yield Y_(obs), that is to say the ratio of theamount of sludge produced to the amount of COD removed by the system,was also calculated.

The results obtained are illustrated by FIGS. 2 a and 2 c, which showthe variation in the load removed as a function of the load applied.These figures show that there are no substantial differences, as regardsthe amounts of COD removed, between the Control line and the Test line.

Referring now to FIGS. 3 a and 3 b, these show the cumulative amount ofsludge produced as a function of the cumulative amount of COD removed,in each of the two lines (the Test line and the Control line) and fortwo different sludge ages. The curves illustrated by these figuresdemonstrate that the amount of sludge produced, expressed on the basisof the amount of volatile suspended solids, is lower in the Test linethan in the Control line. The slope of each of the curves represents thecurrent biomass yield, allowing the results thus obtained to becompared. It will be seen that, for a sludge age of 8 days, the biomassyield obtained in the Control line is 0.4 kg VSS/kg COD, whereas it is0.24 kg VSS/kg COD in the Test line. The observed reduction issubstantial (around 40%). With a sludge age of three days, the apparentyield is 0.44 for the Control line and 0.32 for the Test line, i.e. areduction of 27%. It will be recalled that the only difference betweenthe two reactor lines is the presence of growth medium support materialin the Test line, with a volume fill factor of 20%.

Although at the present stage of the experiments the surprising resultsobtained by implementing the method of the invention cannot beformulated into a complete theory, it is possible however to provideseveral explanations.

Firstly, it should be noted that the observed differences between theresults obtained on the Control and Test lines are clearly due to adifferent metabolism of the microorganisms when they are fixed to theirsupport and set in motion by mechanical stirring and/or aeration:

-   -   it is clear that the fixed bacteria has a residence time in the        reactor that is much longer than the free bacteria.        Consequently, the cell mortality is higher, resulting in a lower        production of sludge. However, this factor cannot by itself        justify a 27 to 40% lower sludge production as mentioned above;    -   the fixed microorganisms and the bacterial flock particles        present in the culture medium of the biological reactor of the        Test line undergo mechanical work due to the stirring and to the        abrasion between the particulate materials, because of        collisions between the particles. It is known that the fixed        microorganisms are structured as a biofilm and the cohesion of        this biofilm is provided by exopolymers synthesized by the        bacteria. Large mechanical stresses contribute to the        destruction of this structure; Maintaining a biological activity        on the material therefore requires a continuous synthesis of        exopolymers by the bacteria. As a result, the synthesis of these        polymers becomes a more important metabolic pathway than the        production of sludge. Since these exopolymers are either        partially biodegradable, or soluble, they are involved in the        abrasion mechanism in the liquid effluent.

A larger reduction in sludge for a greater sludge age, as FIGS. 3 a and3 b show, may corroborate this second hypothesis insofar as the durationof the mechanical stress exerted on the biomass is longer.

It was seen above that the use of support materials for the growth ofthe microorganisms required particular means for retaining thesematerials in the biological reactor chamber. An embodiment of theretaining means thus employed will now be illustrated with reference toFIGS. 4 and 5.

These figures show that this retention device, which is placed in frontof the chute 17 at the outlet of the reactor 13 for the treatedeffluent, essentially comprises a screen 15 inclined to the vertical ofan angle α of preferably between 0 and 30°. The spacing of the bars ofthe screen is determined so as to let the water through, but not themicroorganism support particles. The spacing of these bars is thereforeless than the smallest dimension of the support particles used forimmobilizing the microorganisms. A deflector panel 16 is placed parallelto the screen, upstream of the latter in the reactor 13. Provided at thebase of the screen 15 is an air injection rail 14 for flushing thescreen continuously or intermittently. The combined effect of thisdeflector panel 16 and of the flushing thus produced allows theascending liquid flow to be channeled by an “air lift” effect that alsoentrains the particles of microorganism growth support materials 18(FIG. 5). The flow thus created has two advantages:

-   -   firstly, the particles of support material help to clean the        screen 15; and    -   secondly, the high mechanical stresses exerted on the surface of        the particles of support material in this region improve the        sludge reduction effect observed experimentally and as mentioned        above.

The treated liquid effluent discharged from the biological reactor,passing through the screen 15, is then removed by overflow by means of aspillway to the chute 17.

As regards the microorganism support elements, according to the presentinvention it is possible to use any existing material availablecommercially or able to be manufactured in accordance with theabovementioned characteristics. This material must therefore have thefollowing characteristics:

-   -   one dimension, taken along any axis, of between 2 and 50 mm;    -   a particular surface texture, namely the presence of regions        protected from abrasion (that allow the growth of a biomass,        providing the biological activity) and abrasive regions that        make it possible, in the presence of a high enough level of        turbulence as defined above, to exert friction on the external        surface of the other particles present in the reaction medium.

Thus, by taking into consideration the above-mentioned characteristics,a person skilled in the art will be able to select the types ofmaterials suitable for the operation that has to be carried out. A fewnonlimiting examples of materials that can thus be used are given below.

EXAMPLE 1 Particulate Material.

Microorganisms support elements are formed from granular particles thatcan be obtained from the recycling of plastics, as described, forexample in FR-A-2 612 085. FIG. 6 of the appended drawings illustratesan example of such particles that are in the form of granules having avery irregular shape, with recesses 20 protected from abrasion andprotruding parts 19 that promote abrasion. The size of these granules isbetween 2 and 5 mm and their developed surface area may be between 5000and 20 000 m²/m³.

EXAMPLE 2 Extruded Plastic.

In this case, the microorganism support elements are formed fromextruded and cut plastic materials. FIGS. 7 a and 7 b of the appendeddrawings show end and side views, respectively, of an illustrativeexample of such an element. This element is cylindrical in shape and hasribs 21, 22 provided on its external and internal surfaces respectively.The external ribs 21 allow the abrasion action to take place while theinternal ribs 22 increase the surface area available for colonization ofthe biomass. The size of these support elements may be between 5 and 25mm and their total developed surface area may be between 100 and 1500m²/m³.

EXAMPLE 3 Compression-Molded Or Injection-Molded Plastic.

It is known that there are on the market many types of packing elementsfor columns having the required characteristics for taking advantage ofthe present invention. FIG. 8 of the appended drawings shows, inperspective, three illustrative examples of elements of this type. Theyare generally referred to as rings. Their size may be between 10 and 50mm and their developed surface area may be between 100 and 1000 m²/m³.In the rings illustrated in FIG. 8, the abrasive surfaces may be theedges of the cylinders 24 and the recessed parts 23.

It will be noted that, with this type of material, which ischaracterized in particular by a larger size than the previous ones, theabrasion is also effected by the liquid flow through the internalregions. The rings include internal ribs 25 for colonization by themicroorganisms.

Of course, it remains to be stated that the present invention is notlimited to the illustrative examples described and shown above, ratherit encompasses all variants thereof.

1. A biological reactor for implementing a method for the biological purification of wastewater in hybrid cultures employing microorganisms, at least same of which are fixed to solid support elements set in motion so as to generate turbulence in a reaction medium, the intensity of the turbulence being such that it reduces the production of biological sludge, the biological reactor including a microorganism support retention means positioned upstream of means for removing the liquid effluent leaving the reactor, the microorganism support means further comprising: a screen inclined to the vertical at an angle of between 0 and 30 degrees; wherein a predetermined separation of bars of the screen allow water through the screen but not the microorganism support particles; an air injection rail positioned at the base of said screen and selectively operating continuously or intermittently so as to flush the screen; and a deflector panel located parallel to said screen and located upstream of the latter. 